Once the writers’ strike is over, anyone in the mood to make a new monster movie might consider this beast, described today in the journal Biology Letters. It’s Jaekelopterus rhenaniae, a “sea scorpion” that lived 390 million years ago. Based on a fossil of its enormous claws was found in Germany, scientists estimate it measured 2.5 meters long. It’s the biggest arthropod yet known, a giant among giants. At this period in the history of life, lots of insects, millipedes, and other sea scorpions grew to science-fiction sizes, possibly thanks to the high levels of oxygen in the atmosphere at the time. Our own ancestors–lobe-fin fish–might well have been the hapless prey of this spineless behemoth.

Now–what would happen if some of them fell into an underwater cave, where they lingered for 390 million years until some foolish teenagers pulled away the rocks at the entrance and swam in…?

Living scorpions are not the descendants of sea scorpions. The common names are a little misleading. The official name for sea scorpions is eurypterids. After Thanksgiving I’ll write some more about these critters.

Some comments about the size of the thing got me wondering what the error bars on the 2.5m estimate should be. Are there enough J. rhenaniae specimens that they can scale it properly from a claw alone? Could there be an effect like the one Stephen Jay Gould wrote about (concerning Irish Elk) in which one character grows larger at a faster rate than the overall body?

This thing was really huge, but I don´t know if this is really such a sensation as sizes of 2,5m for the largest sea scorpions are already known for many decades. BTW, if you count leg span, the giant Japanese spider crab Macrocheira kaempferi would be even bigger.

From the article, it seems that the size was NOT attributed to the atmospheric oxygen levels (or at least maybe not):

“Gigantism in Late Palaeozoic arthropods is generally attributed to elevated atmospheric oxygen levels, but while this may be applicable to Carboniferous terrestrial taxa, gigantism among aquatic taxa is much more widespread and may be attributed to other extrinsic factors, including environmental resources, predation and competition.”

The size of several very large prehistoric terrestrial athropods is sometimes attributed to higher oxygen levels (although I suppose there were also many ecological reasons for this), but in the case of the giant sea scorpions, there was surely no connection, because even at a very high oxygen level of the atmosphere, the water will not have a higher oxygen level, especially in the deep.

Yes, atmospheric oxygen will diffuse into the oceans. But Sordes is correct in that increasing the concentration of oxygen in the atmosphere does not increase the amount of oxygen in seawater, as that is dependent upon the ability of the water itself to absorb oxygen. This is in turn dependent on factors other than concentration of oxygen in the atmosphere, such as salinity (more salt = less O2), temperature (lower temperature = more O2) and pressure (higher pressure = more O2). There will be slightly more O2 if the overall atmospheric pressure were higher, but that’s about it. In deep waters, oxygen arrives not by diffusion but primarily by convective transport of large masses of water from the surface downwards, which happens today in the polar regions. At the surface you can get some supersaturation of oxygen (thanks to phytoplankton production of O2), but at depth the dominant process is bacterial heterotrophy, using up oxygen.

Doug #9: It is very unlikely that this critter, which is obviously a chelicerate (and therefore an arthropod), is at all closely related to the anomalocarids or to Opabinia. It’s not clear what the relationship of those weird Cambrian critters is to the arthropods, but whatever it is, it’s not close.